CS-10 keyboard triggering issues

Synth Repair
1

I recently purchased a CS-10 for a very reasonable price. It was working great for a few days and then one day I turned it on and was unable produce any sound except white noise while the filter was in HP mode. In addition the trigger LED was constantly on unless I switched the trigger to EXT. EXT triggering works fine. Also the YM24800 key assigner was not producing an output voltage on TP3 so VCO key voltage remained at 0 and doesn’t produce any sound.

The + & -15 VDC and +10VDC rails are within spec however the + & - 7.5VDC rail is reading 7.0V each. I checked all ICs to ensure they are receiving VDC.

My troubleshooting lead me to find that the clock Osc was not producing a square wave at 9kHz (TP9) so I figured the TC4049 clock Osc IC had failed. This chip is also involved in triggering so I had thought replacing it would certainly help. After replacing the stock Toshiba TC4049P with a Toshiba TC4049BP, the clock Osc started producing a square wave at 8.3kHZ, which is in spec, the filter is working normally again however 2 problems still remain:

  1. The keyboard triggering doesn’t work for every note. The lowest 5 notes and a couple of the higher notes don’t activate the trigger LED and don’t produce the -6.9V triggering signal off Tr1. The rest are triggering as expected.

  2. The key assigner YM24800 is not producing a CV which I’m measuring at TP3 as 0V - because of this, the sawtooth and square waves aren’t producing anything audible when a key is pressed

My suspicion is that the key assigner has failed in addition to the Osc clock. Does this add up? Is it logical that the key assigner failed and took the clock Osc down with it? Or vice versa.

The key assigner is quite pricey and I’m hoping I definitively prove it has failed before trying to source one. Is the key assigner also involved in the triggering circuit as well as for producing the CV for pitch? Is there any other component involved that may have failed and could produce these symptoms? I checked the diodes and all the resistors coming off the key assigner and all seems ok.

Thanks!

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2

Yeah it’s quite possible that the assigner has failed. I think there’s however an alternative these days for this particular chip with the added benefit it will add midi to it. Have a look at Tubbutec

1 Like
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Hello and thank you for your feedback. I appreciate it. I did check the Tubbutec website and found they have available a replacement for the SK series key assigner and the CS series is in the works. I’ll have to reach out them!